Do I Need Planning Permission for a House Extension?

A plain-English guide to what you can build without permission — and when you will need to apply

Buildcostr.ai · July 2026 · 7 min read

The short answer

Many house extensions in England can be built without planning permission under permitted development rights. Whether yours qualifies depends on the type of extension, its size, your property type, and where you live.

This guide covers England specifically. Scotland, Wales and Northern Ireland have their own planning rules which differ from those described here.

What are permitted development rights?

Permitted development (PD) rights are granted by national legislation and allow certain types of building work to proceed without a formal planning application. They exist to reduce the planning system's workload for minor, uncontroversial development.

For homeowners in England, PD rights cover a wide range of works including extensions, loft conversions, outbuildings, solar panels and porches — each subject to specific size and location limits set out in the Town and Country Planning (General Permitted Development) (England) Order 2015.

Permitted development limits for extensions in England

Single-storey rear extensions

• Maximum depth: 3m for terraced and semi-detached houses
• Maximum depth: 4m for detached houses
• Maximum height: 4m (or 3m if within 2m of a boundary)
• Must not extend beyond the side wall of the original house
• Must not cover more than half the original garden

Larger single-storey rear extensions (prior approval)

Under the "larger home extension" scheme, you can build a single-storey rear extension up to 6m deep (semi/terraced) or 8m deep (detached) without full planning permission — but you must apply for prior approval from your local planning authority first. This costs £258 and involves notifying your neighbours, who have 21 days to object. The council then decides whether the impact on neighbours is acceptable.

Double-storey rear extensions

• Maximum depth: 3m from the rear wall of the original house
• Must be no closer than 7m to the rear boundary
• Must not exceed the height of the existing house
• Roof pitch must match the existing house as far as practicable
• Upper-floor windows on the sides must be obscure-glazed

Side extensions

Single-storey side extensions are permitted development provided they are no wider than half the width of the original house and no higher than 4m. Two-storey side extensions always require planning permission — they are not permitted development.

When you will always need planning permission

Conservation areas

If your property is in a conservation area, permitted development rights are significantly restricted. Any extension that would be visible from the highway, any side extension, and any roof alteration requires planning permission. Most London boroughs have large conservation area coverage — check your property before assuming PD applies.

Listed buildings

All works to a listed building — internal and external — require listed building consent in addition to any planning permission. Permitted development rights do not apply to listed buildings. Unauthorised works to a listed building are a criminal offence.

Article 4 directions

A local planning authority can issue an Article 4 direction removing permitted development rights in specific areas. These are common in historic towns, sensitive landscapes and some new-build estates where developers included them as a planning condition. If your area has an Article 4 direction, works that would normally be PD require a planning application.

Flats and maisonettes

Permitted development rights for extensions apply only to houses. Flats, maisonettes and other dwellings within converted buildings have no PD rights for extensions and always require planning permission.

Certificate of Lawful Development — do you need one?

If your extension qualifies as permitted development, you are technically free to build without any paperwork. However, a Certificate of Lawful Development (CLD) is strongly recommended.

A CLD is a formal written confirmation from your local council that your proposed works are lawful. It costs £129 in England and takes 6–8 weeks. The benefits are significant:

• Protects you if a neighbour complains or the council queries the works
• Gives your conveyancing solicitor what they need when you sell
• Provides certainty before you commit to architect and builder fees
• Often required by mortgage lenders

Most architects include a CLD application as part of their service on permitted development projects. It is inexpensive insurance for what is typically a £50,000–£100,000+ investment.

Building regulations — the separate approval you always need

Planning permission and building regulations are entirely separate. You can have PD rights and need no planning permission — but you still need building regulations approval for any structural extension work. Building regulations cover structural design, insulation, fire safety, drainage, ventilation and electrics.

Building regulations approval costs £600–£1,200 for a typical extension and is obtained from either your local authority building control or an approved inspector. Your builder cannot legally start structural work without it. For the full cost breakdown, read our guide to planning permission and pre-construction costs.

What happens if you build without planning permission?

If you build an extension that required planning permission without getting it, your local planning authority can issue an enforcement notice requiring you to demolish the works or restore the property to its original state. Enforcement action can be taken for up to 4 years after completion for residential extensions.

More practically, unpermitted works cause problems when you sell. Solicitors acting for buyers will query works that don't have proper permissions, and many mortgage lenders will decline to lend on properties with unauthorised development. The retrospective planning route (a "retrospective application") is available but not guaranteed to succeed.

Next steps

Once you know whether planning permission is required, the next step is understanding what your extension will cost. A professional cost plan produced from your architectural drawings gives you the full financial picture — build costs, professional fees, VAT — before you commit to the project.

Buildcostr.ai produces a complete UK construction cost plan in 30 minutes from your drawings. See a sample or order yours from £99.

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